Men rated a woman shown in photographs as more sexually attractive if she was wearing red clothing or if she was shown in an image framed by a red border rather than some other colour, says Elliot.

He says although this "red alert" may be a product of human society associating red with love for eons, it also may arise from more primitive biological roots.

Noting the genetic similarity of humans to higher primates, he says scientists have shown that certain male primates are especially attracted to females of their species displaying red.

For example, female baboons and chimpanzees show red colouring when nearing ovulation, sending a sexual signal that the males apparently find irresistible.

"It could be this very deep, biologically based automatic tendency to respond to red as an attraction cue given our evolutionary heritage," says Elliot.

Ratings

The study involved more than 100 men, mostly college undergraduates, who were shown pictures of women and asked to rate how pretty they were, how much the men would like to kiss them and how much the men would like to have sex with them.

Men were shown a woman, with some of the pictures bordered in red and some bordered in white, grey or green.

Even though it was the same picture of the same woman, when she was framed in red the men rated her as more attractive than when she was bordered by another colour.

Men were then shown photographs of a woman that were identical except that the researchers digitally made her shirt red in some versions or blue in others. And once again, the men strongly favoured the woman in red.

The men also were asked, "Imagine that you are going on a date with this person and have $100 in your wallet. How much money would you be willing to spend on your date?" When she was clad in red, the men said they would spend more money on her.

The researchers noted that the colour red did not alter how men rated the women in the photographs in terms of likability, intelligence or kindness - only attractiveness.

The researchers then had a group of young women rate whether the pictured woman was pretty. Red had no impact on whether women rated other women as pretty, they found.